1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
In an electric power steering system, an output from an electric motor is transmitted to a steered mechanism after speed of the output is reduced through a worm shaft and a worm wheel, and thus, torque assistance for steering operation is provided. Although backlash is needed in engagement between the worm shaft and the worm wheel, rattle may be caused by the backlash while a vehicle is traveling.
Therefore, conventionally, an electric power steering system has been proposed in which backlash is removed by elastically urging a bearing, which supports one end of a worm shaft, toward a worm wheel. Another electric power steering system has been proposed in which paired elastic bodies are arranged on respective sides of an inner ring of a bearing in order to suppress backlash of the bearing (for example, US2005/0224278 A1 and US2012/0111657 A1).
In US2005/0224278 A1, an elastic body is supported between a nut that is screwed and fitted to an outer periphery of the other end of the worm shaft, and an inner ring of the bearing in a state in which the elastic body is compressed in an axial direction. In US2012/0111657 A1, the elastic body is supported between a circular plate, which is fixed by a retaining ring to an outer periphery of the other end of a worm shaft in an axial direction, and the inner ring of the bearing in a state in which the elastic body is compressed in the axial direction. Meanwhile, in addition to the structure that supports the elastic body in the axial direction, there is provided a joint mechanism that connects the other end of the worm shaft with a rotary shaft of an electric motor so that torque is transmittable between the other end of the worm shaft and the rotary shaft. Therefore, a structure is complicated.